1. Field of the Invention
The present invention relates in general to transducers utilizing a micromachined sensor and a dedicated electronic read-out circuit with self-test capability, and in particular to transducers having a solid-state capacitive pressure sensor and dedicated high-performance read-out circuit on-chip which electronically creates an electrostatic force for self-testing.
2. Description of Related Art
The monitoring and control of pressure and/or gas flow is of critical importance in a variety of industrial processes, including those associated with semiconductor manufacturing. However, in low-pressure applications such as molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and reactive ion etching (RIE), there is a need to extend the precision of flow control further into the sub-SCCM (standard cubic centimeters per minute) range. Present flowmeters typically require the heating or process gases and offer a full-scale range of about one SCCM and a minimum resolution on the order of 10.sup.-2 SCCM. Such flowmeters lack desired precision for may process applications and are unsuitable for gases which begin to decompose slightly above room temperature.
In recent articles about our research, we reported a new ultrasensitive microflow sensor capable of improving flow resolution by five orders of magnitude. This work is described in the papers cited as References 1 and 2 at the end of this patent application. This microflowmeter is based upon the measurement of pressure to determine the flow rate and does not require the heating of process sensitive gases. The structure is fabricated using silicon micromachining techniques and can be scaled over a wide range of flow and pressure response.
A solid-state micromachined capacitive pressure sensor is described in U.S. Pat. No. 4,815,472, which is assigned to the assignee of the present invention, and hereby incorporated by reference. In that patent, there is described a solid-state pressure sensor made using a single-sided dissolved wafer process, which is suitable for use as the capacitive pressure sensor in the self-testable transducer system of the present invention.
Advances in the electronics used with the micromachined, solid-state silicon single-crystal pressure sensors described in the foregoing patent and our above-mentioned microflowmeter are desired to improve transducer performance. We have recognized that the read-out electronics are a critical part of improving the performance of sensing systems which use capacitive solid-state pressure sensors, as will shortly be discussed in more detail.
Self-testability in micromachined sensors has now also been proposed, but only a few implementations for this desirable concept have been put forward. The self-testability in micromachined accelerometers has been discussed in a few articles, see References 3 and 4. Reference No. 3, for example, discloses a circuit which is intended to self-test a piezoresistive accelerometer by using voltage supply signals of .+-.16 volts, which is three times larger than the usual .+-.five volt supplies provided to miniature transducers.
Read-out electronics form a critical part of the sensing system in capacitive solid-state pressure sensors. In general, there are three basic circuit techniques which have been reported in measuring capacitance: capacitance bridges [see References 5 and 6], relaxation oscillators [see References 7 through 10], and switched capacitors [see References 11 through 13]. Capacitance bridges are based on measuring the charge ratio among several capacitors in a bridge configuration. Although they are simple to implement, the capacitive bridge circuits are highly sensitive to stray capacitance. Furthermore, these circuits typically employ components with high temperature sensitivity, such as diodes or bipolar transistors. Relaxation oscillators work on the principle that capacitors are energy storage elements with charging time constants that are a function of their capacitive value. However, relaxation oscillators also have high temperature sensitivity and the circuits used to measure the frequency are typically slow.
Switched-capacitor circuits are based on converting a charge difference into a voltage using an integrator. A typical circuit 20 is illustrated in FIG. 1, where the positive input of operational amplifier 21 is tied to ground. As the phase 1 signal (.phi.1) and RESET signal (applied to the gate 22 of solid-state switch 24) go high, the adjustable capacitor C.sub.x responsive to some external force or pressure is charged up and the output is set to ground. As the phase 1 signal goes low, the charge difference between the capacitor C.sub.x and the reference capacitor C.sub.ref is integrated across the integrating capacitor C.sub.f by normal operation of the operational amplifier, resulting in a voltage output signal V.sub.out proportional to the total integrated charge difference in a matter of a few tenths of microseconds. The major advantage to this circuit technique is its low sensitivity to parasitics and temperature, and its high speed.
We have reported that our ultrasensitive pressure-based microflowmeter has a potential dynamic range extending over five orders of magnitude, and has a resolution which appears to exceed 16 bits. Although the transducer performance has been characterized [see References 1 and 2], the related issue of providing a correspondingly high-performance electronic read-out for this ultrasensitive microflowmeter has yet to be addressed.
It is a primary object of the present invention to provide for a switched capacitor electronic read-out circuit having a range of at least 10 bits for use with the ultrasensitive microflowmeter described in References 1 and 2.
A related object of the present invention is to handle the problems caused by electrostatic forces generated during the read-out of the microflowmeter's capacitive sensor, so that these forces do not perturb or modify the actual output value.
Still further, it is an object of the present invention, in light of electrostatic forces which can be generated by charging the plates of a capacitive pressure sensor, to provide a circuit which makes self-testing of the capacitive pressure sensor possible.
Another object of the present invention was to develop a simple switched capacitor circuit with a simple high performance operational amplifier capable of being utilized in self-test mode.
Another object is to provide a switched capacitor circuit which includes means for automatically selecting an appropriately sized integrating capacitor to use, depending upon sensed conditions, thereby extending the range of the electronic read-out system.
Yet another object is to provide a dedicated electronic read-out circuit which can be interfaced with one or more standard bus interfaces for communications with a host process controller.
Another object is to provide a switched capacitor electronic read-out circuit which automatically selects an approximately sized integration capacitor from a plurality of choices.